All known lasers comprise the following three fundamental elements: a lasing medium which provides atoms, ions, or molecules that support light amplification, an energy source to excite the medium, and an optical resonator to provide feedback of the amplified light.
One of the most common lasing media in current use in lasers is gas. Solid state lasers are also abundant and employed in industrial application. A common source for exciting the lasing medium is an electrical discharge, though many other means for excitation are also available.
The optical resonators are of various shapes and constructions, as outlined in many publications such as those by M. W. Sasnett in "Comparing Industrial CO.sub.2 Lasers" in Lasers & Applications, September 1984, pages 85-90 or by W. G. Burnell in "Review of CW High-Power Laser Technology," United Aircraft Research Laboratories - East Hartford, Conn., October 1973, UAR-M132.
The optical resonators are constructed so as to provide for a high lasing volume and a high lasing mass. Constructions most common in gas lasers are of the "coaxial" type having a long and narrow shape such as a tube having two mirrors located at both ends, two electrodes located between the mirrors, and the gas being introduced into the tube so that it flows in the direction of the laser beam produced in the resonator.
Such a construction has several disadvantages. The long distance between the mirrors make it difficult to maintain an accurate permanent relative position between the mirrors, as is essential for the accurate operation of the laser. A further disadvantage derives from the large distance between the electrodes. Because the excitation voltage is proportional to distance and pressure, a laser operating with high lasing mass must use a very high excitation voltage, thereby causing many safety and technological problems. Furthermore, the high friction rate of the gas flowing along the walls of the tube increases its temperature and its staying time in the tube, thereby decreasing its lasing capability and requiring the application of a high power gas pump.
In order to overcome these problems and to provide for more efficient lasers, lasers with other geometric constructions have been developed. Once such construction is the so called "cross flow" construction wherein the resonator is defined by two mirrors of high surface area, the electrodes being coplanar with the mirrors, and the gas being flown into the resonator perpendicular to the direction of the laser beam. Such a construction allows for a significant shortening of the time of stay of the gas in the resonator. However, because the gas is not heated homogenously along the resonator the electrical discharge will be higher at those zones where the electrical resistance is lower and the gas temperature is higher i.e., the lower lasing zones.
Another known construction is the "cross beam" type, wherein the electrical discharge and the gas flow are in the same direction and the mirrors are perpendicular thereto. In such constructions the non-homogenity in the temperature of the gas does not interfere with the electrical discharge, however, since the beam advances along non-homogenous thermal zones, excitation is predominant at the warmer zones, the least sufficient zones in respect of lasing.
An additional disadvantage of both the "cross beam" and the "cross flow" constructions is that only a very small part of the gas volume in the resonator is utilized for the production of the beam.
In all known constructions, as described above, the excitation of the lasing medium is not symmetric and therefore a non-symmetric laser beam is produced. Furthermore, the high temperature along the resonator causes non-uniformity in the beam output, thereby imparing the symmetry of the beam's cross section and mode.
The application of conical mirrors in optical resonators is known, as in U.S. Pat. No. 4,164,366 which discloses a resonator formed with optically connected cavities comprising a power extraction cavity and a mode control cavity, means for coupling the two cavities, and a conical reflective surface. The known resonator disclosed therein is a non-symmetric complex system having many components, and thus the assembly and alignment of the resonator is complicated.
European Published Patent Application No. 0100089 discloses a laser having a resonator comprising interalia substantially conical reflective surfaces for the emission of the laser beam. However, the known resonator is of a long and a non-symmetric structure, having a plurality of reflectors thereby infering a complexity in structure and operation.
U.S. Pat. No. 4,025,172 describes a compound unstable resonator comprising a pair of axially disposed rotationally symmetric mirrors and a centrally disposed conical folding mirror. The power extraction cavity is defined to have a generally cylindrical configuration and to lie intermediate the pair of rotationally symmetric mirrors.
German Offenlegungsschrift No. 24 45 597 also describes an unstable resonator. In this configuration, the power extraction cavity is not rotationally symmetric. Similarly, U.S. Pat. No. 4,164,366 shows a resonator having a rotationally symmetric fold mirror and wherein the power extraction cavities are not rotationally symmetric.